The blades and vanes of gas turbine machines operate in an extremely harsh environment with high gas pressures and velocities and temperatures around 1300° C. In order to withstand this environment, the blades and vanes in a combustion turbine are made of high temperature alloys such as nickel-cobalt, are coated with a thermal barrier coating (TBC) such as yttria-zirconia, and, if necessary, are internally cooled to help dissipate heat. The condition of the TBC is critical because spalling, whereby areas of the TBC flake off or separate from the underlying alloy, can lead to vane or blade failure within hours. At present, machines are shut down at regular intervals and inspected, and damaged blades and vanes are then replaced. Condition monitoring of vanes and blades of operating machines until now has not been available, although gas monitoring has been proposed to look for particulates from the thermal barrier coating as an early warning system.
Typically, gas turbine machines have three or four rows of vanes and blades having thermal barrier coatings and are subjected to the most severe conditions. There may be, depending upon the type of machine, about 50 to 100 blades and vanes per row, and up to approximately 200 to 500 total blades and vanes having a TBC. Currently, it is necessary to periodically stop the turbine and inspect all of these components for deterioration of the coating or other defects. It would be desirable to determine the condition of the thermal barrier coating of these components while a gas turbine machine is in operation. Avoiding the need to periodically stop the turbine for inspection reduces downtime and increases turbine efficiency. Similarly, early detection of defects reduces repair costs and outage time, again increasing turbine efficiency. A need exists for monitoring the condition of the thermal barrier coating of blades and vanes within the turbine over time, while the turbine is in operation, to detect changes in the coating and deterioration thereof.
Various methods and systems for detecting and locating defects within a turbine engine or in turbine components have been proposed. Previous work of the current inventors, pending U.S. application Ser. Nos. 09/965,715 and 09/776,818, and expressly incorporated herein by reference, has shown that acoustic monitoring of a combustion turbine can be used to detect the presence of foreign objects in the turbine and to detect wear in the TBC by generating acoustic signals and receiving the signals with an acoustic sensor. As the coating deteriorates, the magnitude and velocity of the acoustic waves changes, indicating the need for service in the turbine.
U.S. Pat. No. 5,445,027 describes a method and apparatus for detecting and locating defects (such as cracks) in a component of a turbine. The method involves using a probe in the interior of the turbine to measure the acoustic spectrum of the turbine, which is then compared with a reference spectrum. Deviations from the reference indicate a damaged blade.
There continues to be a need for methods and apparatus for detecting the deterioration of the thermal barrier coating on blades and vanes in a combustion turbine, to provide an indication of when a turbine needs to be shut down for maintenance.